The upper airway muscles of the nose, mouth, tongue and larynx control respiratory airflow resistance, and contract rhythmically in phase with breathing. Despite their significance, information about the physiological properties of upper airway motor units and their role in the gradation of muscle force is unavailable. - The proposed studies are designed to provide fundamental information about the neurophysiology of the nasal dilator muscle motoneurOn pool in healthy human subjects. The nasal dilator muscle was chosen as the experimental model because it is involved in controlling the breath-by-breath diameter of the nasal airway, it is readily accessible in humans, and it responds to reflex-mediated ventilatory stimuli in a manner analogous to the upper airway muscles of the larynx, tongue and face. The Specific Aims include: l) to estimate the number of motor units in the nasal dilator muscles; 2) to determine if the nasal dilator motoneuron pool behaves in accordance with the Size Principle, as occurs in limb muscle; 3) to determine the contributions of rate modulation and motor unit recruitment to the gradation of muscle force; 4) to determine the relationship between the neural drive to the nasal dilator muscles, and the force produced by these muscles during rhythmic, involuntary contractions; and to estimate the relative contributions of motor unit recruitment and rate modulation to the rhythmic force variations; and 5) to determine the contractile and endurance properties of these muscles. The measurements to be made include muscle force, single motor unit electrophysiology, muscle twitch properties and evoked potentials, and spike-triggered average force of single motor units in conscious human subjects. There is great clinical interest in upper airway muscle physiology because malfunction of these muscles often initiates or exacerbates the obstructive sleep apnea syndrome. This syndrome is associated with hypoxia and respiratory acidosis, acute and chronic hypertension, and often results in death in both infants and the elderly. These studies will contribute to our understanding of upper airway muscle physiology, and will provide the foundation for the development of treatment strategies.